The joining or coating, using a fusible soft solder metal powder-containing paste can be done efficiently, provided, however, that upon heating and before fusion of the metal, the soft solder metal powder containing paste exhibits controlled hot slump; in other words, the metal powder stays substantially where and as the paste was deposited on the substrate. This primary property has been difficult to obtain with conventional metal powder-containing pastes. The instant vehicles and pastes provide such property.
Restriction of flow of any resulting fused metal (that is a lack of general spreading), except into intended joints or to form intended joints as in the joining of electronic components to printed circuit boards or hybrid circuit board to avoid bridging in surface mounted device attachments, are other properties desired. The inventive products here can also provide desired restriction of metal flow on remelting. For most electronic soldering purposes at this time, the composition must be free of corrosive agents such as inorganic salts.
Soft solder pastes, for example, (melting below 400.degree. C., preferably below 330.degree. C.) usually contain tin and lead; a little silver often is alloyed with the tin, particularly for electronic work, e.g., for making microelectronic connections. Soft solder pastes now are experiencing wider acceptance in industry, primarily because pastes are more readily adapted to automated manufacture and screen-printing on a substrate than are the more conventional and manually effected solder bar, separate flux and iron, or solder wire (e.g., one containing the flux in a core within the wire) and an iron. The make-up of prior powder metal-containing paste compositions has been largely influenced by the prior practices particularly in respect of fluxing or surface preparing agents. These have comprised substantial portions of organic acids, e.g., rosin acid, inorganic salt materials, e.g., zinc chloride, ammonium chloride, borax and the like, and rosin-based fluxes.
These soft solder compositions have not been entirely satisfactory in a number of respects. First, because of the presence of inorganic salt fluxes they have tended to be corrosive to the base metal or to introduce water as water of hydration leading to spattering. This problem was alleviated to a great extent with rosin-based flux vehicles. However the rosin-based fluxes or vehicles permit excessive movement of the solder particles away from the deposition site on heating; they also often give rise to sputtering and "dewetting" which leaves a residue as a halo around the solder joint. Their solder pastes have been prone to excessive solder balling, i.e., the formation of discrete minute balls of soft solder. This is highly undesirable and leads to electrical bridging across what should be electrically isolated connections especially on printed circuit boards.
In the preferred embodiment of a soft solder composition an alkali solubilized in a polyhydric alcohol, e.g., glycerine, acts as a fluxing agent and improves solderability. For best results herein, in a soft solder metal powder, a slump control system and an inorganic salt-free fluxing system are present. The compositions hereof are practically noncorrosive and easy to formulate and handle. They are readily adaptable to screen printing. Disadvantages attendant prior compositions, such as spattering due to evolution of water vapor, solder balling and bridging, corrosivity, dewetting, tenacious residue, failure of the solder to melt due to excessive oxidation, etc., are not observed in the metal-bearing pastes here. "Inorganic metal salts" are metal or ammonium salts of inorganic acids Lange's "Handbook of Chemistry", 10th Edition, pp. 209-333.
To better understand the applications of solder pastes, reference may be had to "Technology of Electronic Grade Solder Pastes" Taylor et al, Solid State Technology, September 1981, pages 127-135.
One prior paste composition is disclosed by Knoth U.S. Pat. No. 1,772,952. This paste comprises a solder metal powder, an amine and a binder or vehicle such as vaseline. These compositions do not, however, possess antislump properties which are currently in demand in the electronics industry. U.S. Pat. No. 2,493,372 to Williams dated Jan. 3, 1950 discloses a paste composition including a metal powder, e.g., tin-lead, a salt flux, and a C, H and O compound. Reference may also be had to U.S. Pat. No. 3,954,494 dated May 4, 1976 which discloses a wax-flux composition. U.S. Pat. No. 804,664 also shows a tin-lead metal powder paste in a vaseline--glycerine--inorganic salt flux system.
Another solder paste composition comprising powdered solder, a resin, a salt of an amino acid ester and a solvent is discussed by Melchiors et al in U.S. Pat. No. 3,065,538 dated Nov. 27, 1962.
Another aqueous base solder composition is shown by Johnson in U.S. Pat. No: 3,073,270 dated Jan. 15, 1963.
Also exemplary of prior solder paste compositions is the patent to Petersen et al U.S. Pat. No. 3,925,112 dated Dec. 9, 1975. According this invention, there are provided emulsified self-cleaning soldering fluxes having an aqueous phase including from 2 to 3 pbw (parts by weight) of a water soluble hydrazine salt and an oil phase including from 5 to 50 parts of a mixture of petrolatum and wax. Also provided are water solutions of 2 to 30 pbw of the water soluble hydrazine salt, up to 5 parts of a vegetable gum thickener, a wetting agent and the balance water.
Another prior paste composition is disclosed by Mastrangelo in U.S. Pat. No. 4,273,593 dated June 16, 1981. This paste comprises a solder metal powder and a vehicle, the latter being a hydroxypropyl cellulose blended with glycerine and/or water and further blended with a mixture of polyalkoxyalkanols.
It has now been found that an improved vehicle for the instant powdered metal solder pastes is provided as an essentially inorganic salt free nonaqueous vehicle medium comprising a normally liquid nonaqueous organic liquid as an antislump agent, usually and preferably a polyhydric alcohol system, characterized in that the liquid has a minimum surface tension of 43 dynes/cm (20.degree. C.), and generally from 43 to 65 or higher dynes/cm. Such system provides a slump control property to the metal bearing paste such that when it is heated toward the melting point of the soft solder metal powder, e.g., 118.degree. C. to 325.degree. C., the shape of the deposit is substantially maintained until the metal fuses. Although as will be seen from Table I below, and nonaqueous liquid having a surface tension above 43 dynes/cm. may be used in the vehicles hereof, polyols are the most practical antislump materials and the invention will be described with reference to polyols for convenience.
These vehicles also desirably include a low surface energy material such as a hydrocarbon. The hydrocarbon, which is preferably solid or semi-solid (e.g., grease-like) melts, of course, and wets the substrate around the deposit, but carries no significant amount of the metal particles with it. Although hydrocarbons having melting points above 15.degree. C. are preferred, in certain embodiments normally liquid hydrocarbons such as hexane, cyclohexane, toluene or mixtures thereof, e.g., mineral spirits, may be used. The high surface energy material serves as a binder under heat conditions to hold the particles in position. Also the hydrocarbon is easily cleaned, along with any residue that may be left, away from the joint or surrounding area. The hydrocarbon by itself is incapable of imparting slump control properties to a powdered metal contained therein. In the preferred powdered metal compositions especially adapted for screen printing, no dewetting is observed. Surprisingly, even after melting, the fusible metal resists running from the site of application. Of course, in soft solder compositions there is also need for a fluxing agent. Such agent serves to clean the surfaces of the substrate or joining metal as well as the solder particles, and to inhibit the formation of oxide coating on the solder particles as the temperature is elevated toward melting.
The desirable properties of antislump, anticorrosion and freedom from spattering in the metal-bearing pastes here are not observed in prior compositions known to me. Other additives may desirably be present as will be pointed out below. The instant pastes have no water in them other than insignificant amounts possibly present as an adventitious impurity that might be associated with good quality technical grade ingredients. No water is added deliberately as such or as a hydrate; so it can be said that the instant pastes and vehicles are "nonaqueous" as a practical matter. Also, it has been found that the high surface tension liquid, e.g., a polyol, can be a single component vehicle or a part of a multicomponent vehicle, i.e., including also a hydrocarbon. The substrate on which the paste is deposited has no significant effect on the hot slump resistance.